f(x) = 2x + c, g(x) = cx + 5, fg(x) = 6x + d. c and d are constants. Work out the value of d. 3 marks.

fg(x) = 2(cx+5) + c

= 2cx+10+c                           [1 mark]

fg(x) = 6x+d. fg(x) = 2cx+10+c

6x = 2cx and d = 10+c          [2 marks]

3 = c and therefore d = 13    [3 marks]